This invention relates to a method of making ball grid array columns for use in conjunction with integrated circuit packages.
Area array packaging is becoming increasingly used in integrated circuit packaging. The most prominent technology at present is the ball grid array package wherein solder balls are secured externally of the integrated circuit package and make contact with lead fingers of a lead frame within the package. The solder balls are then connected in standard manner to a printed circuit board or the like.
A problem associated with the use of such solder ball grid array packages is solder ball degradation due to high stresses. A geometrical approach which minimizes the stresses is to build solder columns rather than solder balls since the solder columns are more compliant than the solder balls and therefore absorb more stress with the use of less solder as compared with solder balls. Unfortunately, present technology makes the solder columns difficult to manufacture The prior art formation of such solder columns has involved loading a carrier with small sections of solder wire, reflowing one end of the solder wire to the substrate and then trimming the wire to size.
In accordance with the present invention, solder bumps can be made in a gang manner by extruding the bumps onto the substrate, this being very adaptable to high volume manufacturing. The bumps are formed as columns of solder by extruding the bumps in a gang process from a solder tape directly onto a heated substrate. The substrate may be heated, for example, by a heater placed against the substrate or closely thereto. The volume of the bump is determined by the solder tape thickness and the extrusion geometry. The height of the bump is dependent upon the amount of material available for extrusion, this in turn being dependent upon the solder thickness and the extrusion chamber geometry. The extrusion chamber is designed to clip the column out of any solder tape residue when the extrusion ram bottoms out. The extrusion mold may be optionally heated to improve solder flow, if required.
The procedure for fabricating the solder columns in the form of a ball grid array on a substrate is to provide a reel-to-reel solder tape indexer which will feed a tape of solder material and the tape. The tape is of predetermined thickness to provide, along with the geometry of the extrusion chamber, a predetermined amount of solder. The substrate, which can be the integrated circuit package, is lined up under the extrusion chamber so that the apertures extending through the extrusion chamber line up with the solder ball or column locations on the substrate. A heater is provided to heat the substrate to the flow temperature of the solder or, if the extrusion chamber is also heated, to a temperature sufficient to cause solder flow in conjunction with the heat provided by the extrusion chamber. The tape is placed over the extrusion chamber and over the apertures extending therethrough and a ram then enters the apertures in the extrusion chamber to force the portion of the tape extending over the apertures down through the apertures and onto the substrate. The solder will flow slightly due to the heat provided by the heater either alone or in conjunction with the heated extrusion chamber. After the solder has reflowed on the substrate, the substrate is removed from the heat sources and the solder thereon is allowed to cool and harden. The solder tape can then be refed by the tape indexer with the process repeated on a further substrate. The above described procedure provides solder columns of uniform volume and height and directly attaches the columns to the substrate in one process. Missing columns from the array are eliminated by the extrusion/pinch-off process.
Advantages of the above described procedure are high speed, low cost with unused solder tape being reworkable into additional solder tape. Solder tapes coated with foils of different metallurgies may be used for special bonding processes.